1. Preheat oven to 180°C (350°F/ Gas 4). Heat the oil in a large frying pan. When hot, add the beef and sear on all sides. Transfer to a roasting tin and roast for 10-15 minutes; the meat should be browned on the outside and rare in the centre. Set aside to cool.
2. Slice the beef downwards to give thin wafers of meat. Combine the beef slices, mushrooms, chopped pepper and parsley in a large bowl.
3. Mix the olive oil, lemon juice, vinegar, sugar, pepper, garlic and cream in a bowl. Whisk lightly, pour over the salad and toss well. Serve at once.